Rapid buring propellant charge for automobile air bag inflators, rocket motors, and igniters therefor

ABSTRACT

A rapid burning propellant charge for applications including igniters, launch eject motors, and gas generators for automobile air gags. The propellant charge comprises a reticulated substrate having a quantity of interconnected ligaments and a coating of solid propellant material on the ligaments. In order to provide a large amount of surface area for a fast burn time, interstices are between coated ligaments to define propellant surface area for combustion. In applications where minimum smoke is desired, the reticulated substrate is preferably composed of carbon, graphite, or a non-combustible material, and the solid propellant material is preferably a minimum smoke type.

The present invention relates to solid propellant charges. Uses of thepropellant charges of the present invention include, but are not limitedto, igniters and launch eject motors where it is desired that thepropellant charges burn rapidly for rapid development of heat or thrust.Other uses of the propellant charges of the present invention are as gasgenerators for automobile air bag inflators as well as other apparatuswhere it is desired to produce inflating gas rapidly. Therefore, for thepurposes of this specification and the claims, a "propellant charge" ismeant to include gas generators for air bag inflation systems and otherinflation systems. However, it should be understood that this inventionis not limited to just these uses, but may find uses, for example, asmain propulsion propellant charges for rocket motors.

A typical solid propellant charge includes a fuel such as aluminumparticles and an oxidizer such as ammonium perchlorate which are usuallybound together by a binder such as hydroxy terminated polybutadiene. Thebinder may also act as a fuel. Where the fuel and oxidizer are separatematerials which are mixed together to form the propellant, thepropellant is known as a "composite propellant."

A composite propellant is usually manufactured by blending theingredients into a thick and viscous but still pourable mixture which isthen added to the rocket motor chamber where the mixture is cast andcured into a solid mass of propellant material in position for use. Forsome applications such as igniters and air bag inflators, the propellantmixture may be extruded into a desired geometric shape such as, forexample, pellets for an air bag inflator, as illustrated at 62 in U.S.Pat. No. 4,547,342 to Adams et al.

In a solid propellant charge, burning proceeds in a directionperpendicular to the surface at all times. Thus, in a type of rocketmotor known as an end burner wherein the propellant grain is a solidmass of propellant without a perforation therein, burning is initiatedat the nozzle end and proceeds in a direction toward the head end of therocket. The burning time for an end burner type of propellant grain isrelatively slow compared to those propellant grains which are perforatedlongitudinally usually along their longitudinal center lines. In thistype of grain, burning may be initiated along the entire length of thepropellant grain so that the burning proceeds from the perforationradially outwardly toward the rocket motor case. The burning time for apropellant charge is also determined by the shape of the internalperforation, the shape known as a "tube shape" or "center perforate"being relatively slower burning, for example, than the shape known asthe "internal star shape" of propellant grain.

Tactical weapons such as canister fired missiles may use launch ejectmotors containing solid propellant charges for ejecting missiles out oftheir canisters before their main motors ignite. It is desirable thatthe propellant grain for the launch eject motor as well as thepropellant charge for the igniter for the flight motor thereof be of thesmokeless or minimum smoke type since large quantities of smoke orexhaust including any toxic gas therein may be injurious to theoperators thereof, and the smoke or exhaust may undesirably hindervisibility of the target which visibility must be maintained afterlaunch for control of the missile. However, smokeless or minimum smokepropellants do not usually burn as fast as is normally desired. Further,this type of propellant tends to become soft in the high temperaturestypically encountered or which may be encountered in areas of the worldwhere such tactical weapons may be used. If the propellant becomes toosoft and its physical state is as a result altered such as duringacceleration as the missile is ejected from the canister, the burningproperties of the propellant are accordingly altered resulting possiblyin an inadequate burning rate or possible explosion of the launch ejectmotor.

It has been suggested in U.S. Pat. No. 3,191,535 to Mulloy to prepare asolid propellant which consists essentially of a cellular fuel elementhaving uniform interconnecting spherical voids of a metal or metalalloy, and a propellant material filling the voids.

It has also been suggested in U.S. Pat. Nos. 3,616,841 and 3,946,039 toWalz that form retaining reticulated structures of metal or the like maybe used as solid propellant reinforcement and burning rate modifiers.These Walz patents, which are hereby incorporated herein by referenceand made a part of this specification, describe methods for producingsuch a reticulated structure by using as a pattern a self-supportingreticulated polyurethane or organic foam formed of ligaments to providea substantially homogeneous reticulated structure which may have a widerange of pore sizes, varying from 3 to 125 pores per linear inch, andthe finished foam material is characterized as having ligaments whichare continuous, gas-free or of low porosity, and of integralconstruction.

U.S. Pat. No. 4,321,220 to Camp discloses a method for strengthening apropellant charge by incorporating a support structure in the propellantcharge. The method is disclosed as comprising slowly traversing aflexible perforated material through a propellant lacquer until thedesired loading is obtained. Camp states that the "reinforcing"substrate is perforated or the like so that it is "permeable topropellant decomposition gases which evolve during storage" and shouldalso be strong and have a low density. Although Camp states that the"reinforced" propellant should have a "waffle appearance" so as to have"desirably increased surface area", the disclosure in Camp addresses theproblems of increasing propellant strength, and the "waffle appearance",while a step in the right direction, still does not provide asufficiently rapid burning smokeless or minimum smoke propellant charge.The Walz patents do not disclose increasing propellant charge surfacearea to achieve more rapid burning thereof and do not therefore affordan adequate solution to the problem either.

It is therefore an object of the present invention to provide asmokeless or minimum smoke propellant charge which is rapid burning,i.e., has a fast burn time. "Burn time" refers to the time it takes toburn a specified volume of a propellant charge and varies depending onthe physical configuration of the propellant charge as well as the typeof propellant. On the other hand, "burn rate" perpendicular to thesurface of a propellant is constant for a particular propellantmaterial.

It is another object of the present invention to provide such asmokeless or minimum smoke propellant grain or charge which is alsocapable of withstanding high acceleration forces even when thepropellant material has a tendency to become soft in high temperatures.

One suggestion for solution to the problem of slow burn times forsmokeless or minimum smoke propellants has been to add burn ratecatalysts such as lead and/or copper salts to the propellants to controltheir ballistic behavior, i.e., increase their burn rates. However,because of the toxicity of such additives, it is also desirable toeliminate them from the propellant charges. It is therefore a stillfurther object of the present invention to provide a smokeless orminimum smoke propellant charge with decreased burn time but which doesnot contain such toxic substances.

Igniter propellant material which consists of pellets of boron andpotassium nitrate must be placed in a housing such as a wire basket ortube which is perforated so that the propellant gases from the ignitermay communicate with a propellant grain for ignition thereof. Such wirebaskets or perforated tubes may get blown off during the ignition phaseor get plugged up both of which conditions affect the safety of therocket. It is a further object of the present invention to eliminatesuch a basket or tube for an igniter and thus also reduce the expensethereof.

It is still a further object of the present invention to provide anautomobile air bag inflator which is capable of utilizing a decreasedburn time gas generator propellant charge.

It is another object of the present invention to reduce the hazards to apropellant charge which may result from shock waves or static discharge.

It is still a further object of the present invention to providedecreased burn time of a propellant charge.

It is yet a further object of the present invention to provide a fastburn time propellant charge which is safe, reliable, and non-toxic.

The above and other objects, features, and advantages of this inventionwill be apparent in the following detailed description of the preferredembodiments thereof which is to be read in connection with theaccompanying drawings.

IN THE DRAWINGS

FIG. 1 is a side view of a canister fired missile embodying the presentinvention with a portion of the case broken away;

FIG. 2 is a half cross-sectional view, taken in a longitudinal plane,providing a detailed view of the broken away portion of FIG. 1;

FIG. 3 is a partially cross-sectional view, taken in a longitudinalplane, of the flight motor of FIG. 1;

FIG. 4 is a perspective view of a portion of a reticulated structure fora solid propellant charge embodying the present invention;

FIG. 5 is a sectional enlarged view of a portion of a solid propellantcharge embodying the present invention, including a portion of thereticulated structure of FIG. 4; and

FIG. 6 is a cross-sectional view of a gas bag inflator embodying thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a canister fired missile at 10 whichmay be shoulder launched and which includes a launch eject motor 12 anda flight motor 14. The launch motor 12 is a solid propellant motor whichis used to eject the flight motor 14, which has a length typically offrom about 10 inches to about 6 feet, out of a canister (not shown)after which the flight motor 14 is caused to ignite when it is a safedistance away from the operator. The size of the launch motor 12 isrelated to the flight motor size. For a flight motor having a length of40 inches, the launch motor may have a length of about 4 inches.

Referring to FIG. 2, the launch motor 12 includes a generallycylindrical case 16 and nozzle 18. At 21 is shown an electric match orinitiator commonly know as a squib which is fired by electric lead wires22 which enter the nozzle opening through a conventional nozzle plug 20.The ignitor 24, fired by squib 21, expels hot gases onto the surfaces ofthe launch motor propellant grain illustrated at 28 to initiate burningthereof. After ejecting the flight motor 14 from the canister (notshown), the launch motor 12 is caused to separate therefrom by means ofany suitable separation apparatus, generally illustrated at 30, whichincludes forward closure 31 and a conventional separating piston 23. Theclosure 31 is releasably held to the flight motor case structure 32 bycircumferentially spaced pins 25. Retaining ring 27 retains and alignsthe forward closure in position. Such separation apparatus is of aconventional design and can be constructed using engineering principlescommonly known to those of ordinary skill in the art to which thisinvention pertains and will therefore not be described in further detailherein.

Referring to FIG. 3, the flight motor 14 includes a generallycylindrical case 32 which includes a head end 34 and which extendsrearwardly therefrom to a nozzle 36 which is molded or otherwisesuitably attached to a support member 37 which is in turn sealinglyattached to the case wall by means of lock ring 39 and an o-ring seal41. Contained within the case 32 is a propellant charge 38 which isignited by igniter 40. Disposed between the propellant charge 38 and thecase wall 32 is a layer 42 of suitable insulation and a suitableinhibitor 43. The igniter 40 is initiated by one or more squibs 44 whichare electrically fired by lead wires 45 which pass through a throughbulkhead connector 47. In order to prevent accidental or prematureignition, a suitable conventional ignition interlock 48 is provided toprevent firing of squibs 44 until after the missile 10 has been ejectedfrom the canister (not shown). Wires for a conventional lanyard assemblyare illustrated at 46 and pass through the ignition interlock which alsoserves as a nozzle closure illustrated at 48.

With the expections of the propellant charge 28 for the launch ejectmotor 12 and the igniter 40 for the flight motor 14, the missile 10 isof a conventional design and can be constructed using engineeringprinciples commonly known to those of ordinary skill in the art to whichthis invention pertains. Therefore, the conventional portions of themissile 10 need not and will not be described in any greater detailherein.

Referring back to FIG. 2, in order to prevent the ejection of smoke orexhaust fumes out of the launch motor nozzle 18 which smoke or exhaustfumes may be injurious to the operator and obstruct visibility of thetarget after launch which is necessary to control the missile, thepropellant charge 28 preferably comprises a minimum smoke propellantmaterial such as, for example, a propellant material containing byweight, 5% poly (ethylene glycol), 4% polyfunctional isocyanate, 30%nitrate ester plasticizer, 60% nitramine, and 1% stabilizers (such asN-methyl nitroanaline to scavenge nitrate ester deposition products).

In order to reduce smoke during the ignition phase of the flight motor14, the igniter 40 for the flight motor 14 also contains a minimum smokepropellant material such as, for example, a propellant materialcontaining, by weight, 47% nitrocellulose, 48% nitrate esterplasticizer, 4% processing aids (such as viscosity modifiers), and 1%stabilizers.

Although many propellants may produce water vapor contrails at highaltitudes, propellants which provide high quantities of smoke whenburned at or near sea level (where canister fired missiles are usuallyfired) contain substantial quantities (i.e., 20 percent or more byweight) of chloride producing compounds such as ammonium perchlorate(which produces hydrogen chloride) and/or substantial quantities (i.e.,10 percent or more by weight) of metal such as aluminum. If a propellantcontains more than about 5 percent by weight of a metal salt (usuallyused as a burning rate catalyst), a high quantity of smoke may alsoresult. Therefore, for the purposes of this specification and theclaims, "minimum smoke" propellant or material is defined as apropellant or material which contains, by weight, 0 to 20 percentchloride producing compounds, 0 to 10 percent metal, and 0 to 5 percentmetal salts.

In comparison with other rocket motor propellants, a minimum smokepropellant does not burn very fast. However, the addition of lead and/orcopper salts to the propellant charges for either the igniter 40 or thelaunch motor grain 28 in order to increase their burn rates is notconsidered desirable due to the toxicity of such burn rate catalysts.

In order to decrease the burn time of the propellant charge for theigniter 40 and the launch motor grain 28 without the addition of burnrate catalysts and to eliminate the basket or tube housing for theigniter pellets as well as to provide strength to the propellant chargesso that the minimum smoke propellants may withstand acceleration forcesduring launch, the propellant charges for the igniter 40 and the launchmotor grain 28, in accordance with the present invention, are eachcomposed of a reticulated structure, illustrated at 50 in FIGS. 4 and 5,the ligaments 52 of which are coated with a suitable propellant material54 such that interstices are between the coated ligaments for increasedsurface area as will be described more fully hereinafter. As shown inFIG. 4 wherein reticulated carbon is illustrated, the reticulatedstructure 50 is composed of a multitude of such ligaments 52 which areof generally uniform dimension and interconnected with each other toform voids 55 which are open to each other. For the purposes of thisspecification and the claims, a "reticulated structure" or "reticulatedsubstrate" is meant to refer to a structure or substrate which iscomposed of a multitude of ligaments which are interconnected with eachother to form voids which are open to each other and includes such astructure as described in the Walz patents.

In order to prevent or reduce the production of smoke from burning ofthe reticulated structure 50 in accordance with an aspect of the presentinvention, the reticulated structures 50 for both the launch motor grain28 and the igniter 40 as well as other applications where a minimumsmoke propellant is required are each composed of a non-combustiblematerial, i.e., a material which does not burn at the temperatures andconditions under which the coated propellant is burned, or a minimumsmoke material. Although carbon and graphite may be combustible undersome conditions such as if the propellant mass is oxygen rich, carbonand graphite produce non-smoke producing carbon dioxide when they burnand are thus minimum smoke materials.

Other applications of the coated reticulated structure of the presentinvention may not require that the propellant charge be smokeless or ofa minimum or reduced smoke type in which case it may be preferable thatthe reticulated structure 50 be composed of combustible material suchas, for example, aluminum, boron, beryllium, or copper so that it willalso burn as the propellant material burns to provide increased energy.Thus, it should be understood that the reticulated structure 50 of thepresent invention is not limited to a minimum smoke or non-combustiblematerial.

The ligaments 52 may be sized such that the reticulated structure 50only occupies between about 11/2 and 6% of the volume of a propellantcharge. The coating 54 may be applied to the ligaments 52 by anysuitable means commonly known to those of ordinary skill in the art towhich this invention pertains such as by dip coating or by spraying ontothe ligaments a propellant lacquer prepared by dissolving a propellantmaterial in a suitable solvent such as acetone and then allowing thematerial to dry on the ligaments. The thickness, illustrated at 56 inFIG. 5, of the coating 54 is determined by applying engineeringprinciples of common knowledge to those of ordinary skill in the art towhich this invention pertains in order to achieve desired impulse andother performance requirements. However, in accordance with the presentinvention, the thickness 56 of the coating 54 is such that interstices,illustrated at 58, are between the interconnected coated ligaments todefine propellant surface area 60 for combustion. It may be necessary torepeat the process of dip coating or spraying and then drying severaltimes to allow a build-up of propellant material to the thickness 56desired. The thickness may be such that the interstices 58 are so smallas to be hardly noticeable to the eye so that, taking into considerationthe volume taken up by the reticulated structure 50, a very highpercentage of perhaps 90 to 971/2% of the available volume of apropellant charge is occupied by propellant material but yet the surfacearea 60 for rapid burning may be increased by perhaps on the order of500 percent or more.

Referring to FIG. 6, there is shown generally at 70 a gas generator orinflator assembly according to the present invention for the generationof gas to inflate a vehicle inflatable crash protection bag. Inflator 70has a generally cylindrical external outline and includes a housingconstruction 72 comprising two structural components. The two structuralcomponents comprise an upper shell or diffuser 74 and a lower shell orbase 76 which are joined by three concentric inertia welds shown at 78,80, and 82 to form the housing construction 72 of the inflator assembly70. The three inertia welds are performed simultaneously in a singleinertia welding operation.

The diffuser 74 may be formed by forging with three concentric cylinders84, 86, and 88, each of which cylinders extend downwardly from a commonflat upper wall 90 of the diffuser 74 to form a separate weld interfacewith the base 76. The inner cylinder 84, in cooperation with wall 90 andbase 76, forms a cylindrical igniter chamber 92. The intermediatecylinder 86, in cooperation with the inner cylinder 84, wall 90, andbase 76, forms an inner chamber having the shape of a toroid,specifically, a combustion chamber 94. The outer cylinder 88, incooperation with the intermediate cylinder 86, wall 90, and base 76,forms an outer chamber 96 that also has the shape of a toroid. Cylinders84, 86, and 88 each include a plurality of uniformly spaced exhaustopenings or ports 98, 100, and 102 respectively through which thegenerated or inflation gas flows into a protective air bag (not shown)to be filled. The base 76 includes an interface attachment flange 104which is used to attach the inflator assembly 70 to a vehicle theoccupants of which are to be protected.

Positioned within the igniter chamber 92 is an igniter charge assembly105 comprising a rupturable closed aluminum container 106 containingigniting material 108. Container 106 may be hermetically sealed againstmoisture, has a recess or cavity 110 formed in the bottom 111 thereof,and is retained in chamber 92 by a retaining ring 112. Retaining ring112 has a shape conforming to the bottom 111 of container 106 includingrecess 110 and may be inserted in the end of chamber 92 in press fitrelation therewith. At the top end thereof, container 106 is held inspaced relation with the inner surface of wall 90 by cushion/spacermaterial 114 which desirably may comprise a cerafiber material.

Although various pyrotechnic materials may be employed for ignitermaterial 108, a preferred material is a granular mixture of 25% byweight of boron and 75% of potassium nitrate. This mixture has beenfound to burn with a very hot flame that is suitable for igniting thesolid fuel gas generant material employed in the inflator assembly 70,as described hereinafter.

Extending into recess 110 of container 106 is an initiator 116.Initiator 116, as shown, has a conically shaped lower portion and ismounted in a hole 118 having a mating conically shaped upper portion,the hole 118 being provided at a central location in base 76. Initiator116 is retained in hole 118 by a crimp 120 that is formed in base 76 atthe upper end of hole 118 and which overlaps and engages the conicallyshaped upper portion of initiator 116. Initiator 116 may be aconventional electric squib having a pair of energizing electricalterminals (not shown) that are adapted for plug-in connection toexternal crash sensor means (not shown).

In order to achieve a rapid burn time for a slow burn rate propellantmaterial so that a suitable slower burn rate propellant material may beused for gas generation in accordance with the present invention, agenerally doughnut-shaped propellant charge 122, which includes areticulated structure similar to the reticulated structure 50 of FIG. 4,preferably composed of a minimum smoke material such as carbon orgraphite, or a non-combustible material, and which is coated with asuitable propellant material, preferably a minimum smoke propellant, asdescribed above, is contained within the toroidal combustion chamber 94.

Propellant charge 122 is surrounded by an annular inner screen pack orcombustion chamber filter 124. Inner screen pack 124 may desirablyinclude a layer 126 of coarse screen adjacent to the inner surface ofconcentric cylinder 86. An aluminum washer-shaped retaining ring or disk128 holds the propellant charge 122 and inner screen pack 124 in placeand away from the base 76 during the inertia welding operation.

The internal surface of the base 76 includes a circular rounded ridge130. This ridge 130 serves to reduce the free volume of combustionchamber 94. Additionally, during functioning of inflator assembly 70,ridge 130 provides support for the retainer disk 128.

In the outer toroidal chamber 96, an aluminum deflector ring 144 isprovided. Deflector ring 144 is formed with an inwardly directed curvedflange 146 at its upper end and has a plurality of uniformly spacedexhaust openings or ports 148 adjacent the bottom end thereof. Ring 144has a length at least as long as concentric cylinder 86 and ispositioned in embracing relation with the latter with flange 146 inpress fit engagement with the outer surface of cylinder 86 at the innerend thereof and engaging weld flash 140 at the outer end thereof. Alsoincluded in the toroidal chamber 96 is an outer screen pack or filter150. Screen pack 150 may desirably include a coarse layer 152 adjacentthe inner surface of cylinder 88.

Functioning of the inflator assembly 70 begins with an electrical signalfrom a crash sensor (not shown) to the initiator 116. The initiatorfires into and pierces the closed aluminum container 106 that holds theigniter material 108. The igniter material 108 burns and bursts throughthe walls of the container 106 and flows through the exit openings 98 inthe inner cylinder 84 and into the toroidal combustion chamber 94. Thehot igniter gases ignite the propellant charge 122 which releasesinflator gases. These gases flow through the inner screen filter pack124 and radially outward through the combustion chamber exit openings100. The screen filter pack 124 serves to cool the inflator gases and toremove particulate residue therefrom. As the gases exit the combustionchamber openings 100, they are turned downward by deflector ring 144where they strike flashing 140 from the intermediate cylinder inertiaweld 80. The flashing 140 serves to interrupt the gas flow which helpsto further remove particulate matter from the exhaust gases. Theinflation gases then flow radially outward through openings 148 in thedeflector ring 144 and up into the annular space between deflector ring144 and outer screen pack 150, through the latter, and finally radiallyoutward through the exit openings or port holes 102. The outer screenpack 150 serves to further cool the exhaust gases and remove particulatematter.

More detailed information concerning such an inflator is contained inthe aforesaid patent to Adams et al, which is hereby incorporated hereinby reference and made a part hereof. It should be recognized, of course,that pellets of the prior art rather than the coated reticulatedstructure of the present invention is disclosed as composing thepropellant charge in Adams et al.

In order to prepare a propellant charge in accordance with the presentinvention, a reticulated substrate 50 is first prepared by the methoddescribed in the Walz patents or by any other suitable method. Then theligaments 52 of the reticulated substrate 50 are coated with a suitablesolid propellant material 54 as previously described by dip coating,spraying, or by any other suitable coating method which may requireseveral coats as previously discussed. The coat or coats of material 54are then allowed to dry on the ligaments 52 while interstices 58 aremaintained between the coated ligaments to define propellant surfacearea 60 for combustion.

The resulting propellant charge may be installed as a rocket motorpropellant or igniter, a gas generate for an air bag, or in othersuitable applications utilizing principles commonly known to those ofordinary skill in the art to which this invention pertains. For example,a propellant charge embodying the present invention for an igniter suchas igniter 40 may be suitably attached, by use of a set of threads (notshown) or bonded to a suitable structural support (not shown) of arocket motor, and a propellant grain such as grain 28 embodying thepresent invention may be suitably bonded to the case such as case 16 ofa rocket motor.

As an example, an embodiment of the present invention has been tested bydip coating a mixture of butanetriol trinitrate (BTTN) andnitrocellulose (NC) prepared in an acetone solution onto reticulatedcarbon. The following is data from samples prepared by dip coating a50/50 (weight/weight percent) BTTN/NC solution in acetone (50%) ontoreticulated carbon which has 45 pores per inch:

    ______________________________________                                                Weight of Weight of   Burn Time at 1,000 psi                          Sample No.                                                                            Carbon (g)                                                                              BTTN/NC (g) (in./sec.)                                      ______________________________________                                        1       0.0702    0.5535      7.87                                            2       0.0822    0.6565      6.211                                           3       0.0895    0.7568      5.84                                            4       0.0796    0.6095      13.69                                           ______________________________________                                    

A typical burn time at 1000 psi for the tested propellant material in aconventional solid form without interstices therebetween is betweenabout 0.2 and 0.4 in. per sec. Thus, the test results show that a burntime several times greater than the typical burn time of a propellantmaterial may be achieved by a propellant charge which is composed of thepropellant material and which embodies the present invention. Samples ofthe tested propellant charge were prepared and burned in a window bombwherein large amounts of subsurface combustion (i.e., combustion in theinterior of the coated reticulated structure) were observed.

If the reticulated structure 50 is non-combustible, the remainingstructure after combustion may serve as a combustion stabilizer by thedamping of pressure or sound waves and may also serve as a means fordisrupting gas flow vortex formation. In addition, a carbon reticulatedstructure may be grounded to the motor case to prevent the hazards ofstatic discharge.

Since the total surface area available for burning in a coatedreticulated structure embodying the present invention is so large, sucha coated reticulated structure is provided to achieve a fast burn timeno matter what the burn rate of the propellant material coated thereon.

It is to be understood that the invention is by no means limited to thespecific embodiments which have been illustrated and described herein,and that various modifications thereof may indeed be made which comewithin the scope of the present invention as defined by the appendedclaims.

What is claimed is:
 1. A propellant charge comprises a reticulatedsubstrate having a quantity of interconnected ligaments and a coating ofsolid propellant material on said ligaments, said coating has athickness such that interstices are between coated ligaments whichinterstices define propellant surface area for combustion.
 2. Apropellant charge according to claim 1 wherein said reticulatedsubstrate is composed of a material selected from the group consistingof a minimum smoke material and a non-combustible material, and saidsolid propellant material is a minimum smoke propellant.
 3. A propellantcharge according to claim 1 wherein said reticulated substrate iscomposed of a material selected from the group consisting of carbon andgraphite.
 4. A method of making a propellant charge comprises the stepsof:a. preparing a reticulated substrate having a quantity ofinterconnected ligaments, b. coating the ligaments with a solidpropellant material, and c. maintaining the thickness of the coatingsuch that interstices are between coated ligaments which intersticesdefine propellant surface area for combustion.
 5. A method according toclaim 4 wherein the step of coating the ligaments comprises dip coatingthe ligaments in a propellant lacquer prepared by dissolving the solidpropellant material in a solvent, and further comprises allowing the dipcoated ligaments to dry.
 6. A method according to claim 4 wherein thestep of coating the ligaments comprises spraying propellant materialwhich is dissolved in a solvent onto the ligaments and allowing thespray coated ligaments to dry.
 7. A method according to claim 4 furthercomprises composing the reticulated substrate from a non-combustible orminimum smoke material, and composing the solid propellant material froma minimum smoke propellant.